How do atoms of iron react with molecules of oxygen in
air to form a compound?

Steel wool burns in air with a very bright white light, giving
off a large amount of energy, and white smoke with is iron oxide
in very fine particles. The steel wool changes color to a more
gray appearance which is the iron oxide. It is proven that the
steel wool has changed into a compound because you can no longer
ignite the steel wool after it has burned.

A different reaction occurs more slowly to give the orange/brown
color of rust that you may see on various iron objects.

Iron Oxide: Determine the
formula of a compound formed by the reaction of iron and oxygen.

Solution:

First examine the electron arrangement of the iron and oxygen
atoms.

Symbol

Atomic No.

Bohr diagram

Group No.

Lewis Dots

Fe

26

2 - 8 - 14 - 2

8 B

3 *

O

8

2 - 6

6

6

*Iron does not follow the rules in determining the Lewis diagram.
This is a special case and unless you are an expert, you would
not be expected to figure this out by yourself.

Write the Lewis symbols for each atom. See graphic on the
left.

Determine the numbers of electrons which the atoms will lose
and gain by applying the Octet Rule. Fe
loses three electrons**. Oxygen gains two electrons to have an
octet.
**Octet rule does not apply to iron.

Fe Atom

Fe Ion

O Atom

O Ion

26 p+

26 p+

8 p+

8 p+

26 e-

lose 3 e-

23 e -

8 e-

gain 2 e-

10 e-

0 charge

+3 charge

0 charge

-2 charge

The ionic bond between ions results from the electrostatic
attraction of opposite charges.

The final formula of iron oxide is Fe2O3.

Why does iron oxide have subscripts in the formulas?

The Octet Rule is the basis for the predictions about the
charges on ions. A compound is not stable unless the number of
electrons which are lost equals those gained. The electrons gained
or lost on each atom may be different. The exchange of electrons
continues until there are no left over electrons and no empty
spots. This sometimes results in several atoms being required.
The subscripts are written to show how many of each kind of atom
is involved.

SUMMARY PRINCIPLE OF IONIC COMPOUNDS

An ionic compound is formed by the complete transfer of electrons
from a metal to a nonmetal and the resulting ions have achieved
an octet. The protons do not change. Metal atoms in Groups 1-3
lose electrons to non-metal atoms with 5-7 electrons missing
in the outer level. Non-metals gain 1-4 electrons to complete
an octet.

OCTET RULE:

Elemental atoms generally lose, gain, or share electrons with
other atoms in order to achieve the same electron structure as
the nearest rare gas with eight electrons in the outer level.

The proper application of the Octet Rule provides valuable
assistance in predicting and explaining various aspects of chemical
formulas.